Police said they received a 911 call shortly before 5 p.m., and numerous officers responded. A text alert was sent to students and staff about the situation, instructing them to stay away from the area.

The caller said there was a "man with a gun strapped to his chest and carrying a rifle," Bermudez said. "The caller also said that a shooting in the Administration Building had occurred and asked 'where were the police.'"

The Administration Building and some surrounding buildings near the campus' central mall were evacuated. Streets leading to the Administration Building were blocked, and dozens of police officers — some wearing helmets and armored vests and others armed with assault weapons — were seen on campus with a helicopter hovering overhead.

Tucson police and Pima County bomb squads also rushed to the scene but found no signs of any explosives, Bermudez said.

"No one was found nor were there signs that a shooting had taken place in building, and there have been no reports of anyone injured," he said.

Bermudez declined to comment when asked if the call had been a hoax, saying only that the case was under investigation.

A few hours after the search began, the department's Twitter site said: "UAPD has cleared the Admin Bldg. No one was found, no injuries or signs of a shooting. The Stu Union IS OPEN."

A shooting at the University of Arizona's nursing school in October 2002 left three professors dead before the gunman killed himself.